This invention relates to a driving device for printing magnets employed in printing apparatus such as line printers.
In some of such printing magnet drive devices, in order to reduce the number of drive circuits, the printing magnets are divided into a plurality of groups, and the magnets in each group are driven by one drive circuit. Hereinafter, a description will be made with reference to a case where the printing magnets are divided into two groups, that is, a group for odd columns, and a group for even columns, for convenience in description.
Shown in FIG. 1 is one example of a printing magnet drive device in which the printing magnets are divided into a group for odd columns and a group for even columns, and the printing magnets in each group are driven by one drive circuit.
Printing magnets 1 and 2 provided respectively for the odd columns and even columns and adapted to energize printing hammers (not shown) are connected to the terminals of a power source (not shown) through control transistors 3 and 4 reverse-current preventing diodes 5 and switching transistors 6. Furthermore, a series circuit of a diode 7 and a zener diode 8 which form a flyback absorption circuit is connected in parallel to each of the printing magnets 1 and 2. The aforementioned control transistors 3 and 4 are alternately rendered conductive by means of a shuttle switching control circuit 9 with the aid of an inverter 10. In other words, in the odd column printing, the transistor 3 is rendered conductive, while in the even column printing the transistor 4 is rendered conductive.
In the circuit thus organized, the saturation voltages across the collectors and emitters of the control transistors 3 and 4 are varied with the variations of currents flowing therebetween, and therefore the voltages applied to the printing magnets 1 and 2 cannot be maintained unchanged. This is one of the drawbacks accompanying the conventional printing magnet drive device. In other words, when the number of printing magnets 1 and 2 to be driven is changed, the saturation voltages of the transistors 3 and 4 are changed. When the voltages applied to the printing magnets 1 and 2 are changed, the flight time of the printing hammer, that is, the time interval which elapses from the beginning of energization till the hammer strikes a printing type is changed, as a result of which printing quality is lowered.
Furthermore, the number of zener diodes 8 forming flyback absorption circuits connected across the printing magnets 1 and 2 is relatively large, and therefore the conventional printing magnet drive device has problems to be solved with respect to the number of components and in manufacturing cost.